In a mass spectrometry, liquid samples are often used as the object to be analyzed. An example is an analysis with a liquid chromatograph mass spectrometer (LCMS), in which a sample dissolved in a solution is separated into components by the liquid chromatography. Then, the components are sequentially sent to the mass spectrometer, which carries out the mass analysis of each component.
For the mass analysis of a liquid sample, a liquid sample ionizer using an assist gas (or nebulizing gas) is employed as an ion source for generating ions to be analyzed. In this ionizer, a liquid sample ejected from a liquid supply pipe is nebulized (i.e. broken into droplets) by a strong stream of gas, called an assist gas or nebulizing gas, flowing along the outer surface of the liquid supply pipe. The gas also functions as a carrier and drier of the droplets, and often as an electrifier of the droplets.
In general, liquid sample ionizers carry out the ionization with the assist gas at roughly atmospheric pressure. The ions generated thereby are introduced into the mass spectrometer unit, the inner space of which is maintained in a high vacuum state.
FIG. 6 schematically shows the construction of a mass spectrometer 10 using an assist gas for ionization. The mass spectrometer 10 includes an ion source 41 for generating ions at roughly atmospheric pressure and a mass spectrometer unit 13 enclosed in a vacuum chamber 12.
The ion source 41 is mainly composed of a gas transport pipe 14 and a liquid supply pipe 15. The gas transport pipe 14 is cylindrical at its center and tapered at its front end. Located at the center of the tapered end of the ion source 41 is a gas supply passage 17 with an ejection port 16 for ejecting the assist gas. The gas transport pipe 14 has, on its side, a gas inlet 18 and a gas supply conduit 19 for introducing the assist gas into the gas supply passage 17. The gas supply conduit 19 is connected to the gas supply passage 17 within the gas transport pipe 14.
The liquid supply pipe 15 is inserted into the gas supply passage 17 of the gas transport pipe 14 to form a duplex pipe structure. The liquid supply pipe 15 extends through the hole 20 formed at the rear end of the gas transport pipe 14 and leads to an external source of the liquid sample, e.g. the liquid chromatograph in the case of an LCMS. The front end of the liquid supply pipe 15 is located close to and slightly sticking out from the ejection port 16.
The liquid sample flowing through the liquid supply passage 21 of the liquid supply pipe 15 is sent to the ejection port 16 of the gas supply passage 17. At the ejection port 16, the assist gas coming from the gas supply passage 17 blows away the liquid sample located at the front end of the liquid supply passage 21, nebulizing and drying the liquid sample. The nebulized liquid sample forms a spray, which is directed toward the pore 22 formed in a wall of the vacuum chamber 13. Thus, the ejection port 16 functions as a spray nozzle for spraying the sample. The sprayed droplets of the liquid sample are dried and atomized before they enter the pore 22.
After passing the pore 22, the sample is detected by the mass spectrometer unit 13, which generates signals used for mass analysis. The mass spectrometer unit 13 may be a quadrupole, an ion trap, or any other type selected in accordance with the purpose of the analysis.
There are several types of ion sources that use the assist gas. FIGS. 7A–7D show examples of conventional ion sources using the assist gas.
FIG. 7A shows an ion source using the electrospray ionization. In this ion source, a high voltage source 25 is connected to the liquid supply pipe 15 to electrify the liquid sample located at the front end of the liquid supply pipe 15 by applying a high voltage to the liquid supply pipe 15. The electrified liquid sample is drawn in a predetermined direction by a potential gradient to form a spray directed frontward from the ejection port 16. Each droplet in the sprayed sample becomes smaller in size as a result of the drying process and/or the electrostatic repulsions due to its own charge, and finally turns into ions. In principle, the electrospray ionization does not necessarily require an assist gas. Under practical conditions, however, it is necessary to efficiently perform the spraying and drying processes when a considerable amount of liquid sample is used. Therefore, even in the case of the electrospray ionization, it is common to insert the liquid supply pipe 15 into the gas supply passage 17 and simultaneously supply the assist gas and the liquid sample from the gas supply passage 17 and the liquid supply pipe 15, respectively.
FIG. 7B shows an ion source using the sonic spray ionization. In this ion source, the high voltage is not applied to the liquid supply pipe 15. Instead, the liquid sample 21 is electrified into ions by the friction between the droplets (i.e. liquid sample) ejected from the liquid supply pipe 15 and the assist gas ejected from the gas supply passage 17.
FIG. 7C shows an ion source using the atmospheric chemical ionization. This ion source includes a heater 26 for producing a gas sample by heating the liquid sample flowing through the liquid supply passage 21. The heater 26 also heats the assist gas flowing through the gas supply passage 17. The heated assist gas and the heated gas sample are simultaneously ejected to dry the gas sample. The dried gas sample is then ionized by an electric discharge from the needle-shaped high voltage electrode 27 to which a high voltage is applied with the high voltage source 25.
FIG. 7D shows an ion source using the atmospheric photo-ionization. This ion source includes an excitation light source 28 in place of the high voltage electrode 27 in FIG. 7C and ionizes the gas sample by irradiating the excitation light 29.
As shown in FIG. 8, in the ion source 41 with the liquid supply pipe 15 inserted into the gas supply passage 17, the liquid supply pipe 15 is supported only by a cantilever structure at the hole 20 formed at the rear end of the gas transport pipe 15. This structure, however, does not assure that the liquid supply pipe 15 is always coaxial with the gas supply passage 17 of the gas transport pipe 14; it may allow the displacement of the central axis of the liquid supply pipe 15 from the central axis of the gas supply passage 17. For example, the displacement may be caused by the self-weight of the liquid supply pipe 15, the use of a liquid supply pipe 15 having an originally poor linearity, or a varying flow of the assist gas.
If the displacement occurs, the traveling direction of the ions contained in the gas sample sprayed from the ejection port 16 is also displaced from the center of the pore 22. This leads to a biased distribution of the ion density, which in turn causes a decrease in the amount of the ions passing through the pore 22. As a result, the intensity of the detection signal of the mass spectrometer unit 13 decreases, which deteriorates the sensitivity of the mass analysis.
One of the simplest methods of solving the above-described problem is to manually adjust the position of the ejection port 16 with respect to the pore 22 and find the best position at which the detection sensitivity is maximized.
Another method of maintaining the coaxiality of the liquid supply pipe 15 and the gas supply passage 17 is to fit a bush into the space between the gas transport pipe 14 and the liquid supply pipe 15.
FIG. 9A is a longitudinal sectional view of the front part of an ion source 42 having a bush 31 for holding the liquid supply pipe 15 within the gas supply passage 17, and FIG. 9B is the cross-sectional view at line A–A′ in FIG. 9A.
The bush 31 is fitted into the gas supply passage 17 of the gas transport pipe 14 with a slight gap (e.g. about 5 μm) between the outer circumference of the bush 31 and the inner surface of the gas supply passage 17. The bush 31 has a hole 32 formed at its center, and the liquid supply pipe 15 is fitted into the hole 32 with a slight gap (e.g. about 5 μm) between the inner surface of the hole 32 and the outer surface of the liquid supply pipe 15. Leaving such gaps is necessary to allow the liquid supply pipe 15 and the bush 31 to be removable for cleaning and other maintenance work.
From the working point of view, the existence of the gaps means that the above-described fitting is a “loose fit”, not a “close fit”, as specified in the Japanese Industrial Standards as JISB0401.
In addition to the hole 32, the bush 31 has four slits 30 for allowing the assist gas to pass through. The slits 30 may be replaced by holes or other types of openings.
The Japanese Patent Publication No. 2003-517576 discloses another method of maintaining the coaxiality of the liquid supply pipe 15 and the gas supply passage 17. According to this method, the liquid supply pipe 15 is surrounded by plural pieces of gas transport pipes 33 having the same shape and size, through which the assist gas is supplied.
FIG. 10A is a longitudinal sectional view of the front part of the ion source 43 having the liquid supply pipe 15 surrounded by plural pieces of gas transport pipes 33 for supplying the assist gas, and FIG. 10B is a cross-sectional view at line B–B′ in FIG. 10A.
The above-described three methods address the problems that the liquid supply pipe 15 is displaced and, accordingly, the gas supply passage 17 and the liquid supply pipe 15 are out of the coaxial position. But they cause some other problems.
In the first method, i.e. the manual adjustment of the position of the pore 22 and the ejection port (or nozzle) 16, the adjustment work is very troublesome. Moreover, if the adjustment is insufficient, it is impossible to obtain an adequately high degree of reproducibility of the mass analysis.
In the second method using the bush 31 for holding the liquid supply pipe 15 as shown in FIGS. 9A and 9B, the position of the bush 31 with respect to the inner surface of the gas supply passage 17 is determined by fitting. Similarly, the position of the liquid supply pipe 17 with respect to the inner surface of the hole 32 of the bush 31 is also determined by fitting. In principle, any fitting structure must have a minimal gap between the two elements concerned. This gap inevitably allows the elements to have a room for displacement, so that their position cannot be completely fixed.
This means that the displacement can be as large as the sum of the two gaps, i.e. the first gap between the outer surface of the bush 31 and the inner surface of the gas supply passage 17 and the second gap between the inner surface of the hole 32 of the bush 31 and the outer surface of the liquid supply pipe 15, and the sum will be at least 5 to 10 μm. This displacement is not negligible with respect to the gap between the gas transport pipe 14 and the liquid supply pipe 15, i.e. the distance between the inner surface of the gas supply passage 17 and the outer surface of the liquid supply pipe 15. Such a displacement may cause the detection signal of the mass spectrometer to be weakened or unstable since the ion density varies.
According to the third method shown in FIGS. 10A and 10B, the liquid supply pipe 15 is surrounded by plural pieces of gas transport pipes 33 having the same shape and size, through which the assist gas is supplied. In this structure, the outlets of the gas transport pipes 33 are separated from the outlet of the liquid supply pipe 15 by the thickness of the wall of the gas transport pipe 33. This separation reduces the amount of the assist gas acting on the liquid sample located at the front end of the liquid supply pipe 15, so that the liquid-sheering force of the assist gas significantly decreases. As a result, the liquid sample cannot be fully broken into minute droplets, and the atomization, transport and drying of the liquid sample cannot be adequately performed. This causes an inadequate ionization and accordingly weakens the detection signal of the mass spectrometer. To avoid such a problem, it is necessary to compensate for the shortage of ions by increasing the flow rate of the assist gas to compulsorily promote the ionization.
In view of the above-described problems, an object of the present invention is to provide a mass spectrometer having an ion source constructed so that the gas supply passage for supplying the assist gas and the liquid supply pipe for supplying a liquid sample are maintained in the coaxial position, and the liquid supply pipe is hardly displaced with respect to the gas supply passage.